In situ Hybridisation
Fluorescence in situ hybridisation (FISH) is a widely used method that has been used for decades. Due to its high specificity and sensitivity, FISH can analyse structures at subcellular level or even identify and localise individual cells in large cell aggregates.
The detection method is based on hybridising fluorescent-labelled oligonucleotides with cell´s own nucleic acids. The probe may detect DNA or RNA thus allowing not only information about the presence of a particular gene, but also the gene product (RNA) - and thus gene activity - can be detected.
For the analysis of microorganisms short, fluorescent-labelled oligo probes are used, which hybridise to the cell´s ribosomal RNA. The high copy number of ribosomal RNA per cell allows extremely sensitive detection. Due to the high sensitivity of fluorescence in situ hybridisation, analyses of rare and non-culturable samples are also possible, allowing a wide range of analyses (e.g. marine microbiology, analysis of microbial colonies in biofilms or biogas plants, microbiome analyses, etc.).
In addition to the subcellular resolution of RNA in cells, also a quantitative measurement of RNA in cells via in situ hybridisation is possible.
Fluorescence probes for FISH analysis
For fluorescent-labelled oligonucleotide probes, stable and high-intensity dyes from the entire spectrum of visible light can be used (see fluorescent dyes). Thus, suitable combinations can also be found in autofluorescent environments or for multiplex analyses.
Depending on the assay, different degrees of labelling are available:
The classic FISH probes are labelled with one fluorescent dye at the 5'-end of the oligonucleotide.
A nearly twofold increase in sensitivity may be achieved in in situ hybridisation experiments (DOPE-FISH) by using double labelled probes. Due to the additional sensitivity, this approach can also be used for very rare targets.
The biomers.net dopeProbes are labelled with an identical fluorophore at the 5´- and 3´-end of the oligonucleotide.
An advancement of these are tetraProbes, which have up to four dye molecules on one oligonucleotide and are therefore highly sensitive. Here, the fluorescent dyes are coupled to the oligonucleotide backbone. The signal intensity of the hybridised probe nearly increases linear with the number of fluorophores per oligo. According to this tetraProbes may be two times brighter than dopeProbes and four times brighter than monoProbes under identical conditions.
In addition to the additional light, the insertion of several dye molecules also enables new possibilities for multiplex assays ("MiL-FISH"): Through suitable mixtures of dyes on an oligonucleotide, new colour combinations can be generated and used for specific recognition.
Using alternative coupling strategies enable diverse and flexible ways to insert modifications into oligo sequences.
Of particular note is the technology of internal click modification of nucleic acids. The coupling occurs via the heterocyclic pyrimidine bases 5-octynyl-dC, 5-octynyl-dU or 5-ethynyl-dU. These modified bases are cytidine and thymidine nucleoside analogs and are introduced into the growing oligo sequence at the appropriate position during synthesis.
The actual binding of the marker molecule (e.g. fluorescent dyes) occurs post-synthetically, with the azide-labelled markers reacting with the alkyne of the modified, clickable pyrimidines via the copper-catalysed click reaction.
This form of internal coupling can be used also, for example, for multiple fluorescence-labelled oligo probes, such as the so-called tetraProbes.
In contrast to the commonly used attachment to the 2´-group of the sugar, the click reaction via 5-ethynyl-dU allows a nearly direct modification of the base.
The internal click modification via the heterocyclic pyrimidine bases 5-octynyl-dC, 5-octynyl-dU or 5-ethynyl-dU is not only used in the synthesis of tetraProbes. Rather, they enable new opportunities to label oligonucleotides several times with identical biomolecules.
Using these so-called multiProbes, for example, oligos with nine internal fluorescent dyes are conceivable.
1. A Straightforward DOPE (Double Labeling of Oligonucleotide Probes)-FISH (Fluorescence In Situ Hybridization) Method for Simultaneous Multicolor Detection of Six Microbial Populations. Behnama F, Vilcinskasb A, Wagnera M, Stoeckerb K; Appl. Environ. Microbiol. (2012), vol. 78 no. 15 5138-5142.
2. MiL-FISH: Multilabeled Oligonucleotides for Fluorescence In Situ Hybridization Improve Visualization of Bacterial Cells. Schimak MP, Kleiner M,Wetzel S, Liebeke M, Dubilier, Fuchs BM; Appl. Environ. Microbiol. (2016), vol. 82 no. 1 62-70.
An enormous increase in sensitivity is achieved by the use of HRP probes. Here, instead of a dye, the enzyme horseradish peroxidase (HRP = horseradish peroxidase) is coupled to the oligonucleotide probe. After hybridisation of the oligos to their target sequence in cell, horseradish peroxidase (HRP), a 45 kDa large enzyme, can convert specific dyes and thereby leads to an amplification of the light signal at the binding site.
The combination of HRP probes and the tyramide signal amplification system (TSA) leads to 10-20-fold increased signal intensities in comparison to fluorescein monolabelled probes. This "Catalysed Reporter Deposition - Fluorescent in situ hybridisation" (CARD-FISH) method represents an excellent tool for quantitative detection of microorganisms.
During the synthesis, an oligonucleotide, usually 18 - 25 bases long, is modified with an aminolink at the 5’-end. This primary amino function is coupled to a reactive bi-functional crosslinking reagent, resulting in an activated oligonucleotide, which can react in a further step with a free amino function of the horseradish peroxidase molecule. Thus, a stable covalent coupling is achieved. The appropriate choice of reaction conditions and excess of reagents leads to 1:1 connection between oligonucleotide and HRP.
Purification and isolation of the HRP oligonucleotide is best achieved by using polyacrylamide gel electrophoresis (PAGE).
Especially with regard to a reliable distinctness of closely related species - for example in pathogen diagnostics -
PNA probes are clearly superior to "normal" oligos due to the increased binding specificity. Due to the peptide backbone, PNA are extremely resistant to nuclease digestion, which distinguish them from natural nucleic acids. PNA oligos have a higher binding affinity, so that even very short PNA oligonucleotides guarantee good specificity. PNA probes are therefore optimal for long-term routine studies.
Links für Probe Design
For the design and application of probes in the field of in situ hybridisation we suggest the following links:
The SILVA rRNA database is a project of Max Planck Institute for Marine Microbiology, Bremen. SILVA database is the world´s largest collection for ribosomal RNA sequences for all three domains of life (Bacteria, Archaea and Eukarya) and thus it is one of the first addresses for taxonomic and phylogenetic questions. In addition to specially developed software for aligning, handling and analysis of sequence data, the website offers a comprehensive labor and troubleshooting collection for fluorescence in situ hybridisation.
Ribocon provides useful bioinformatic services and solutions for industry and academies in the area of environmental, clinical and molecular microbiology. Especially with microbial diversity analysis, phylogenetic studies and in the design of custom probes, Ribocon team is highly experienced.
The online server probeBase is maintained by the Department of Microbial Ecology at the University of Vienna. The online resource enables the targeted search of FISH and microarray probes and PCR primers by name, sequence or target organism. In addition, a detailed documentation of the probes is available.
The Department of Microbiology at the Technical University of Munich presents a collection of informative laboratory protocols and helpful links.
1. Single-molecule fluorescence in situ hybridization: quantitative imaging of single RNA molecules. Kwon S; BMB Rep. (2013),46(2):65-72.
2. Fluorescence In situ Hybridization: Cell-Based Genetic Diagnostic and Research Applications. Cui C, Shu W, Li P; Front Cell Dev Biol. (2016); 4: 89.
3. Applications of fluorescence in situ hybridization (FISH) in detecting genetic aberrations of medical significance. Bishop R; Bioscience Horizons (2010), Volume 3, Issue 1, Pages 85–95.